Field of the Invention
The present application generally relates to image processing and, more particularly, to an image processing technique capable of performing image quality adjustment processing while taking the difference between a photopic vision and a scotopic vision into consideration.
Description of the Related Art
When a user views a display (such as a display screen or a projector screen) in a dark environment, it is conventionally performed to lower the luminance of the display apparatus in such a way as to suppress the glare. Further, it is conventionally feasible for a display device to change a display mode thereof to a cinema mode to realize a low contrast display with low luminance and low color temperature. Further, to lower the resolution in the cinema mode, it is conventionally known to softly adjust the border enhancement.
The above-mentioned correction processing is performed in such a way as to suppress the glare according to eye property or fit to the color temperature whose content has been generated. Further, the above-mentioned correction processing is performed to enable a user to clearly recognize an image even when the image is viewed in a bright environment.
The phenomenon that the working ratio between two visual cells (i.e., cone cell and rod cell) is variable depending on the brightness is generally known eye property. The cone cell is mainly active in a brighter environment and the rod cell is mainly active in a darker environment. The above-mentioned change between the cone cell and the rod cell is generally referred to as “Purkinje”.
The cone cell is capable of recognizing the three primary colors: red, green, and blue. The rod cell is capable of recognizing monochrome colors only. Further, the cone cell is different from the rod cell in the central frequency of the sensitivity. More specifically, if the rate of the scotopic vision increases in a darker environment, the color density and the color temperature are differently recognized due to Purkinje transition.
As discussed in Japanese Patent Application Laid-Open No. 2006-285063, Japanese Patent Application Laid-Open No. 2007-248936, and Japanese Patent Application Laid-Open No. 2007-248935, it is conventionally known to correct the color temperature and the color density according to the Purkinje transition. More specifically, as discussed in Japanese Patent Application Laid-Open No. 2006-285063, Japanese Patent Application Laid-Open No. 2007-248936, and Japanese Patent Application Laid-Open No. 2007-248935, the conventional systems perform image processing in such a way as to increase the color temperature and the color density if an environmental luminance and an averaged picture level (APL) are lower than predetermined levels. The APL is an averaged value of the gradation number of image data included in a single frame.
Further, the Bartleson-Breneman effect is known as another eye property, according to which the luminance at which a viewer feels that the brightness reaches a predetermined level is different between a darker environment and a brighter environment. As discussed in Japanese Patent Application Laid-Open No. 2010-72141, it is conventionally known to change a gamma curve according to the Bartleson-Breneman effect with reference to an external environmental luminance. More specifically, the system discussed in Japanese Patent Application Laid-Open No. 2010-72141 uses a gamma curve that enhances the contrast when the external environmental luminance is bright and uses a gamma curve that lowers the contrast when the external environmental luminance is dark. Further, as discussed in Japanese Patent Application Laid-Open No. 2010-79064, it is conventionally known to change the luminance of a screen according to the Bartleson-Breneman effect.
However, as described above, the difference between the cone cell and the rod cell not only influences the recognition of colors (which is referred to as “Purkinje transition”, as described above) but also influences the recognition of contrast levels. The techniques discussed in Japanese Patent Application Laid-Open No. 2006-285063, Japanese Patent Application Laid-Open No. 2007-248936, and Japanese Patent Application Laid-Open No. 2007-248935 cannot correct the contrast itself and therefore cannot obtain satisfactory image quality although the color can be corrected and the gain value can be limited.
Further, it is known that the dynamic range of an eye is widened when the external environment is dark. For example, a contrast ratio of 1000:1 is sufficient when the external environment is bright. On the other hand, the contrast ratio required when the external environment is dark is a contrast ratio of 10000:1. In other words, when the external environment is dark, it is necessary to enhance the contrast by expanding the darkness of a dark section. However, according to the technique discussed in Japanese Patent Application Laid-Open No. 2010-72141, the contrast rather decreases when the external environment is dark.
Further, according to the technique discussed in Japanese Patent Application Laid-Open No. 2010-72141, the screen luminance is decreased when the external environment is dark, and therefore the contrast decreases. In other words, the adjustment processing performed in this case is opposite to the processing performed in such a way as to enhance the contrast when the external environment is dark. Further, recently, a viewer tends to be positioned closer to a display apparatus that has a large screen. Therefore, the viewer is greatly influenced by a displayed image. The influence of the displayed image is comparable to or rather greater than the influence from the environment. In the technique discussed in Japanese Patent Application Laid-Open No. 2010-72141, the influence of the screen luminance is not taken into consideration. Therefore, the system will lower the contrast when a bright image is displayed even when the external environment is dark.